This invention relates to a method of making for transluminal delivery a stent of working radius R from a tube of radius r and wall thickness T, smaller than R, the method comprising the steps of removing material from the tube wall with the tube at radius r, or substantially r, over the full wall thickness T to leave the tube wall penetrated in a multiplicity of separate cut lines, in a pattern which permits the tube to expand to radius R; expanding the tube to radius R; and performing at least one manufacturing step on the tube at radius R.
The invention also relates to stents which can be made in accordance with such methods.
It is well-known to make self-expanding stents for placement in bodily lumens by selecting a tube of Nitinol shape memory alloy, having a diameter appropriate for transluminal delivery, taking a length of the Nitinol tube corresponding to the desired length of the stent, mounting the tube length in a jig, and using a laser cutting device, under the control of a computer, to cut in the wall thickness T of the tube length a pattern of a multiplicity of full wall thickness cut lines parallel to the longitudinal axis of the tube length. All the cut lines are quite short in proportion to the tube length. All the cut lines are mutually parallel and have an ordered spacing throughout the length of the tube length. The cut lines are arranged in bands around the circumference of the tube length.
It is known, and advantageous, to select, for the band at each end of the stent, a cut length greater than the length of the cut lines present in the bands spaced from both ends of the stent. Other patterns of cut lines of different length can be adopted, to vary the mechanical properties of the stent along the length of the stent, as required by the particular task which the stent is to perform when placed in the body.
The cut lines of each alternate band are co-linear. The cut lines of each adjacent band are, in the circumferential direction, halfway between two adjacent cut lines of the adjacent band either side. Each band of cut lines overlaps with the cut lines of the adjacent band. In this way, the provision of the bands of cut lines enables the tube length to expand radially, with the material of the tube wall between the cut lines deforming and each cut line transforming into a diamond-shaped window in the wall of the tube.
For shape memory materials, it is necessary to impose on the tube length a xe2x80x9cmemoryxe2x80x9d of a shape which it is desired that the tube length should have, in its functioning as a stent, after placement in the bodily lumen. Thus, for Nitinol materials, it is known to expand the tube length, after laser cutting, over a mandrel, to bring it to the diameter which it is desired it should assume when placed in the bodily lumen. Held in that diameter, the tube length is then brought to a temperature sufficiently high to allow the molecular movements within the metal matrix which constitute the memory of the material. When the tube length is cooled, and the mandrel removed, the tube length remains in its expanded radius configuration. It can be compressed down to its original radius and captured within a sheath, for installation on the distal end of a delivery system, for transluminal delivery.
In many applications of such self-expanding stents, high flexibility of the stent, after placement in the bodily lumen, is required. That flexibility is achievable with Nitinol. However, there are applications when the stent has to be delivered through a tortuous bodily lumen, in which case, flexibility of the stent tube length, in its original radius, is also desirable.
An enhancement of flexibility, of the stent in its expanded diameter configuration, has been provided by the present applicant in its stent designs, by parting the material separating two adjacent diamond windows of a circumferential stenting band as described above. Thus, in one convenient arrangement, a parting line is made at two out of every three junctions between adjacent diamond openings in stenting band. The cut lines and parting lines are made by a laser device, so that the width of each cut line and parting line is very small, typically less than 0.06 mm.
The present inventors have found out that self-expanding Nitinol stents as described immediately above, while flexible in their expanded deployed configuration, still lack flexibility in the compressed configuration in which they are transluminally delivered, and that the degree of flexibility is not always sufficient for optimum delivery along tortuous lumens. It is therefore an object of the present invention to facilitate transluminal delivery of such stents by giving the stents an enhanced flexibility when in the compressed, small radius, configuration.
The characterising features of one aspect of the present invention are set out in claim 1 below.
Expressed very simply, material is removed from the tube length, when the tube length is at its large radius configuration, so that the tube length when compressed down to its small radius configuration for transluminal delivery exhibits a plurality of windows. It is these windows which endow the tube length with enhanced flexibility when in the compressed configuration.
For the purposes of understanding the present invention, it is important to appreciate the inter-relationship between the window cutting step and the step of the giving the tube length a memory of the configuration it is required to take up. Laser cutting the starting length of tube is a precision operation in which the computer controlled laser cutting device needs a detailed programme and a fixed starting point of reference on the tube length. It would of course be a simple modification of the laser cutting programme to require the laser to make the transverse parting lines necessary for the subsequent creation of the desired windows. However, one still has the task of bringing the tube length from its original radius r to its end radius R, there to give it a memory of this large radius configuration. Once the parting lines and windows have been laser cut into the wall of the tube length, only a few remaining bridges between adjacent stenting bands prevent the tube length from falling into pieces. Such an entity has less resources to survive expansion on a mandrel from small radius r to large radius R. Accordingly, with the present invention, the designated scrap portions, corresponding to the windows, are not broken away until after the tube length has been expanded to its large radius R.
Choosing not to make the parting lines in the initial laser cutting operation has repercussions for manufacturing efficiency, as follows. The step of expanding the tube length to its large radius has the consequence that the reference point for,the laser cutting equipment is lost, and the expansion on the mandrel gives the stent precursor an individual configuration which varies from tube length to tube length, so that a second computer controlled laser cutting step, in order to part the scrap portions from the material of the tube length, would appear to require fresh calibration. Instead, the present applicant arranges for each scrap portion to be parted from the material of the tube length, at the large radius R as a manual operation. For the time being, this step is not automated.
Nevertheless, steps can be taken to facilitate the manual operation, notably by having the laser, in the initial cutting programme, make blind cuts through the wall thickness of the tube length, which are transverse to the basic cut lines, but which fall short of extending all the way between two adjacent cut lines defining an intervening scrap portion. These blind cuts serve to define the parting lines at which each scrap portion will be parted from the tube length later. For example, if the laser cuts partly, but not wholly through, the strut radial thickness between two cut lines, it has proved to be relatively easy for an operative, using forceps, to pull out each scrap portion held only by a residual frangible portion at the blind end of each blind cut.
Self-expanding stents are known, which exhibit a plurality of stenting bands, arranged end to end along the length of the stent, and each joined to the adjacent stenting band by a bridging band. These bridging bands are often more flexible than the intervening stenting bands, but less effective in resisting tissue stenosis. With the present invention, the scrap portions can be found at locations in the stent which are serving as bridges between adjacent stenting zones, with removal of selected scrap portions reducing the number of bridges between two adjacent stenting zones, and thereby increasing flexibility between these two adjacent stenting zones.
An alternative construction of self-expanding stent sees a continuous helical stenting zone running from one end of the stent to the other, with a helical bridging zone coterminous with the stenting helix. Analogously, the scrap portions can correspond to bridging struts in the bridging helix between adjacent turns of the stenting helix, with the removal of these scrap portions again tending to increase flexibility between adjacent turns of the stenting helix.
Regardless what pattern of stenting zones and bridging zones the tube length exhibits, it will normally be the case that the laser cutting operation creates a pattern of cells in the wall of the tube length, the unit cell being characteristic of the stent and defining its performance.
It is normal to perform polishing steps on self-expanding Nitinol stent precursors, when all cutting operations have been completed. A polishing step is of particular significance for the stent precursors of the present invention, because of the need to remove any rough fracture surfaces where the scrap portions have been removed after laser cutting.